1. Field of the Invention
The present invention relates to a production process of a thin film, and an optical device and, more particularly, to a process of forming a fluorine-containing thin film very susceptible to plasma damage, at a low temperature by a sputtering method of depositing a thin film on a substrate mounted on a substrate holder by sputtering of a target.
2. Related Background Art
In the fields of optics heretofore, vacuum vapor deposition methods of heating a film forming source in vacuum by an electron beam or the like to evaporate it and deposit evaporated particles on a substrate have mainly been used for forming optical thin films such as antireflection films, mirrors, and so on.
In general, the antireflection films, mirrors, etc. are comprised of either a material with a low refractive index such as magnesium fluoride or the like and a material with a high refractive index such as zirconium oxide, tantalum oxide, titanium oxide, or the like, or a multilayer film as a combination of these materials, or the like; layer construction, film thicknesses, etc. are adjusted in various ways, depending upon optical performance required.
The evaporation methods are deposition methods that are simple in system configuration, capable of forming a film at a high speed on a large-area substrate, and excellent in productivity, but they have difficulties associated with controlling the thickness with a high accuracy and with developing automatic systems. Furthermore, films formed at a low substrate temperature have problems of insufficient strength of the film, which can be readily damaged, low adhesion between the film and a substrate, and so on.
However, there has been a growing demand for the increase in production efficiency in recent years. Thus, demands for coatings by sputtering, which are advantageous in terms of labor-saving production steps, stability of quality, film quality (adhesion, film strength) and so on, as compared with the vacuum evaporation methods, have also been increasing for these optical thin films.
When thin films of oxide dielectrics of ZrO2, Ta2O5, TiO2, and so on are formed by the sputtering method, thin films with a low absorption and high refractive index can easily be obtained.
The sputtering system commonly used heretofore for production of thin films is a parallel plate magnetron sputtering apparatus. In this apparatus, a target as a material for a thin film and a substrate mounted on a substrate holder are placed so as to face each other in a vacuum chamber. A plasma is generated to sputter the target. Target particles driven out of the target by sputtering are deposited on the substrate. This is a simple film forming method excellent in highspeed film formation, large-area film formation, target lifetime, and so on.
Meanwhile, there are known sputtering systems other than the parallel plate type systems.
Japanese Patent Application Laid-Open No. 6-17248 describes a proposal of an off-axis type sputtering system in which the substrate or the target is rotated by 90 from the configuration of the parallel plate sputtering system with the target and the substrate facing each other.
Furthermore, Japanese Patent Application Laid-Open No. 5-182911 describes a proposal of a facing target sputtering system in which surfaces to be sputtered face each other with a space therebetween, a magnetic field is generated in a direction perpendicular to the sputtered surfaces, and a thin film is formed on the substrate placed beside a space between the targets.
Japanese Patent Application Laid-Open No. 8-167596 describes a proposal of a plasma processing system and the like in which at least one mesh plate for separation of a plasma is disposed between a plasma generating chamber for generating a plasma and a plasma processing chamber for housing a substrate to be subjected to film formation, the mesh plate is provided with a plurality of apertures, and the diameter of the apertures is not more than two times the Debye shielding length of the plasma.
In recent years, systems using the ArF excimer laser light source and the F2 excimer laser light source have been developed as optical devices used for precision processing. There is an increasing demand for fluorine-containing films of high quality and high durability, which are used in optical parts (or optical components) in optical systems.
Under such circumstances, the inventors have first investigated a method of forming a fluorine-containing film of aluminum fluoride, magnesium fluoride, or the like, using the parallel plate type sputtering system.
For example, a method of forming a fluoride thin film by sputtering is disclosed in Japanese Patent Application Laid-Open No. 4-289165. This sputtering method is for forming a film of an alkali earth metal fluoride, such as MgF2 or the like, by use of a mixture of an inert gas, such as Ar or the like, and a fluorine-based gas, such as CF4 or the like.
There is also a known method of DC sputtering using a metal target and a mixture of an inert gas of Ar or the like and a fluorine-based gas of CF4 or the like, as disclosed in Japanese Patent Application Laid-Open No. 7-166344.
However, when a film of a fluoride material is formed on a lens or the like by introduction of a reactive gas of NF3 or F2 gas or the like, a cathode sheath voltage will accelerate negative ions of fluorine, and fluorine compounds sputtered from the target and high energy particles thus generated will be incident on the film to physically damage it or vary its composition. Furthermore, there can be damage due to the negative ions, such as undesired etching of the substrate instead of a desired formation of a film on the substrate, depending on the species of the target material and the sputtering conditions. Furthermore, the ions and electrons accelerated by the ion sheath may damage the film formed on the surface of the substrate, raise the temperature of the substrate, or increase optical absorption of the film. Particularly, for the ultraviolet light with a high energy, the optical transmittance will be significantly lowered.
The prior art thin film production methods are hardly satisfactory in these aspects.
An object of the present invention is to provide a process of producing a thin film having a high transmittance for the ultraviolet light, particularly, for the vacuum ultraviolet light and being capable of effectively utilizing the light, and also to provide an optical device.
Another object of the present invention is to provide a process of producing a thin film that suffers little damage from negative ions, positive ions, and electrons, and also to provide an optical device.
An aspect of the present invention is a process of producing a thin film, which comprises the steps of:
providing a vessel;
placing a target such that a surface to be sputtered of the target surrounds a discharge space;
placing a substrate on a side of an opening of the space such that the substrate faces an anode disposed so as to close another opening of the space surrounded by the target;
supplying a sputtering gas and a fluorine-containing gas into the vessel; and
supplying a dc power or a power obtained by superimposing pulses with reversing polarities on the dc power, between the target and the anode,
wherein a discharge is induced in the discharge space to sputter the target, thereby forming a fluorine-containing thin film on the substrate.
In the present invention, it is desirable to supply the sputtering gas from the anode side into the discharge space and to supply the fluorine-containing gas from the exterior of the discharge space into the vessel.
In the present invention, it is desirable to supply a reducing gas from the anode side into the discharge space.
In the present invention, it is desirable to supply water from the exterior of the discharge space into the vessel.
In the present invention, it is desirable to use a rare gas as the sputtering gas.
In the present invention, it is desirable to use at least one of a fluorine gas, nitrogen fluoride gas, carbon fluoride gas, sulfur fluoride gas, or hydrofluorocarbon gas, as the fluorine-containing gas.
In the present invention, it is desirable to supply a gas selected from hydrogen gas, deuterium gas, hydrocarbon gas, and ammonia gas, from the anode side toward the discharge space.
In the present invention, it is desirable that the anode""s surface is formed of the same material as the target and that the anode has a number of gas discharge holes.
In the present invention, it is desirable to place a magnetic shield of a magnetic material having a number of holes between the substrate and the target.
In the present invention, it is desirable that the target be comprised of a metal comprising at least one of Mg, Al, La, Nd, Th, Li, Y, Ca, or Gd.
In the present invention, it is desirable to produce, as the fluorine-containing thin film, a film of magnesium fluoride, aluminum fluoride, lanthanum fluoride, neodymium fluoride, thorium fluoride, lithium fluoride, yttrium fluoride, calcium fluoride, or gadolinium fluoride.
In the present invention, it is desirable to produce a thin film under such conditions that the electron temperature Te of the plasma is not more than 3 eV, the electron density is not more than 2xc3x97108 electrons/cm3, and the difference between the potential of the plasma and the floating potential of the substrate is not more than 2 V, on the surface of the substrate.
In the present invention, it is preferable to use calcium fluoride as the material of the substrate.
In the present invention, it is desirable to monitor the voltage applied to the target and to control the supply rates of the reactive gases so that the voltage applied to the target becomes approximately constant.
In the present invention, it is desirable to set the frequency of the pulses within the range of 1 kHz to 500 kHz.
In the present invention, it is desirable to maintain the partial pressure of water within the range of not less than 1xc3x9710xe2x88x923 Pa and not more than 1xc3x9710xe2x88x921 Pa and the partial pressure of hydrogen gas not less than 5xc3x9710xe2x88x922 Pa during the film formation.
In the present invention, it is desirable to form a thin film comprising magnesium fluoride as a main component and containing oxygen in a content of not more than 5 wt % and MgO in a content of not more than 1.5 wt %.
In the present invention, it is desirable to form a thin film comprising magnesium fluoride as a main component and containing a rare gas in a content of 1 wt % to 10 wt %.
Another aspect of the present invention is an optical device comprising an optical system comprising an optical part obtained by forming a fluorine-containing thin film on a substrate by the above-stated thin film production process, in combination with a laser light source for generating an ultraviolet light.